In order to solve depletion of fossil energy and earth-environmental problems caused by using fossil energy, research into alternative clean energy sources such as solar energy, wind energy, and hydro energy that are recyclable and clean has been actively conducted.
Among them, an interest in a solar cell directly converting solar light into electric energy has significantly increased. Here, the solar cell means a cell generating current-voltage using a photovoltaic effect that the cell absorbs light energy from the solar light to generate electrons and holes.
Currently, an n-p diode type single-crystalline silicon (Si) based solar cell having photoenergy conversion efficiency higher than 20% may be manufactured and actually used in solar power conversion, and there is a solar cell using a compound semiconductor such as gallium arsenide (GaAs) having conversion efficiency higher than that of the n-p diode type single-crystalline silicon (Si) based solar cell. However, since these inorganic semiconductor based solar cells require a very highly purified material for high efficiency, a large amount of energy is consumed in purifying a raw material, and expensive processing equipment is required during a single crystallization process or a thinning process using the raw material, such that there is a limitation in lowering manufacturing cost of the solar cell, thereby blocking large-scale use of the solar cell.
Therefore, in order to manufacture the solar cell at low cost, cost of a core material used in the solar cell or the manufacturing process of the solar cell should be greatly reduced, and research into a dye-sensitized solar cell (DSSC) and an organic solar cell that may be manufactured using an inexpensive material and process has been actively conducted as an alternative to the inorganic semiconductor based solar cell.
However, in the case of an organic solar cell using a conductive polymer, efficiency is still about 8% (Advanced Materials, 23 (2011) 4636), and in the dye-sensitized solar cell, in the case of using a liquid electrolyte, the maximum efficiency is about 12 to 13% (Science 334, (2011) 629), and in the case of using a solid type hole conductor, efficiency is still low (7 to 8%). Even in the case of an inorganic-organic hybrid solar cell in a form in which inorganic semiconductor nanoparticles and a hole conductive polymer are coupled in a structure of the dye-sensitized solar cell, efficiency thereof is still about 6% (Nano Letters, 11 (2011) 4789).
Therefore, the development of a solar cell capable of having excellent efficiency enough to replace the single-crystalline silicon (Si) based solar cell according to the related art has been urgently demanded.